The present invention relates generally to light controlling systems. In particular, the present invention is directed to improved electronic flash apparatus.
Electronic flash apparatus is known in the art in which the flash of light produced by a flash tube is automatically terminated after a predetermined total quantity of light has been received from the scene being illuminated. In one particular type of electronic flash apparatus, a flash terminating switch is connected in series with the flash tube. When a light flash is to be produced, both the flash tube and the flash terminating switch are switched to a conductive state. When an exposure control circuit has received the predetermined quantity of light, the flash termination switch is switched to a non-conductive state, thereby terminating the flash.
One specific type of electronic flash apparatus of this general type is shown in FIG. 2 of U.S. Pat. No. 3,727,100 by Kuraishi et al. and in U.S. Pat. No. 3,809,954 by Engelstatter. In this specific type of electronic flash apparatus, a flash trigger switch (generally a silicon controlled rectifier (SCR)) is used to trigger the flash tube and the flash termination switch. Contacts and a capacitor are connected to the control electrode (gate) of the flash trigger switch. When the contacts close, the capacitor discharges, thereby switching the flash trigger switch to a conductive state.
The flash trigger switch is connectd in series with a second capacitor and the primary winding of a transformer. The secondary winding of the transformer is connected to apply an ignition signal to the flash tube. When the flash trigger switch is switched to a conductive state, the second capacitor discharges through the flash trigger switch and the primary of the transformer. The voltage pulse induced in the primary of the transformer is applied to the flash tube to turn the flash tube on.
The control electrode of the flash termination switch is also connected to the flash trigger switch. When the flash trigger switch is switched to a conductive state, the flash termination switch is likewise switched to a conductive state.
One problem which is encountered with electronic flash apparatus of this type is that the switching time required to switch the flash termination switch to its conductive state is much less than the time required to trigger the flash tube into conduction. The flash termination switch is typically a semi-conductor switching device such as an SCR which has a switching time of about 1 microsecond. The time required to ionize the gas in the flash tube may be 20 microseconds or more. Since the voltage of the anode of the flash termination switch is at a low value and because the flash tube is not yet in conduction, the flash termination switch can turn back off before the flash tube turns on.
To overcome this problem a capacitor discharge circuit is sometimes connected to the anode of the flash termination switch. This discharge circuit provides anode-to-cathode current in the flash termination switch to hold the flash termination switch on unitl the flash tube is ionized and the flash tube current begins to flow. The disadvantage of this capacitor discharge circuit is that it requires a number of additional components. In particular, the circuit usually required two resistors to form a voltage divider network to charge the capacitor and a third resistor connected between the capacitor and the anode of the flash termination switch. These additional components increase the cost of the apparatus. In addition, the capacitor discharge circuit requires a relatively large voltage to be applied to the capacitor.
In another form of hold-on circuit, a capacitor is connected between the anode of the commutation switch and the gate of the flash termination switch. The capacitor supplies current to the gate of the flash termination switch to hold the flash termination switch on until the flash tube begins to conduct. This technique, however, has not been fully satisfactory. The gate current to the flash termination switch is dependent upon the voltage of the cathode of the flash tube because the commutation capacitor is connected between the cathode of the flash tube and the anode of the commutation switch. A more reliable hold-on circuit which is not dependent on the voltage of the cathode of the flash tube and which uses a minimum of components is desired.